LAS VEGAS -- This thirsty, sprawling gambling oasis is laying claim to vast reserves of underground water throughout Nevada in a bold move that its adversaries call one of the greatest attempted water grabs in history and a menace to the fragile environment of the desert.

The application by the Las Vegas Valley Water District to gain control of water supplies in an area twice the size of Maryland is a classic western struggle over the region's most precious resource. But because of the potential threat to rare plants and wildlife hundreds of miles away, the 146 Las Vegas applications for underground supplies from 26 Nevada water basins have implications far beyond the state's borders.

Las Vegas developers, backed by Nevada's gambling industry, say the new water is needed for this booming desert metropolis, which during the last two drought years has received only half of its normal annual rainfall of four inches.

But ranchers, conservationists, federal agencies and rural politicians from the three gigantic counties where Las Vegas would take the water warned at an informal hearing held here last week by the Interior Department that approval of the applications could dry up natural springs in the Death Valley National Monument in California, kill rare fish species that have survived since the Ice Age and destroy verdant valleys throughout the West.

Under Nevada law, applications can be approved by the state water engineer alone, and approval is expected after hearings that probably will be held early next year. But it is widely agreed that state approval is likely to face a federal court challenge by the Nevada counties or federal agencies that have protested the applications. Most of the applications affect federal lands.

Supporting the opposition from the Nevada counties was Inyo County in California, where in the early decades of the century, Los Angeles tricked farmers in the fertile Owens River Valley into surrendering their water rights.

Greg James, the Inyo County counsel, pointed out that Owens Valley became a barren dust bowl that now is a major source of airborne pollution in Southern California and warned that the same thing could happen in Nevada.

"These applications could produce 26 Owens Valleys," said Steve Bradhurst, a planning consultant for Nevada's Nye County and coordinator of opposition to the Las Vegas applications.

Nearly a decade ago, Bradhurst led a coalition of ranchers and environmentalists in a successful effort to prevent deployment of the MX missile in Nevada. But he believes that odds against victory are longer now.

"No one predicted that we could stop the MX, but we had {former senator} Paul Laxalt with us in that fight," said Bradhurst. Laxalt, with close ties to then-President Ronald Reagan, helped to kill desert deployment of the MX, which subsequently was placed in existing Titan missile silos.

But Las Vegas and the gambling industry control the levers of political power in the water battle. With more than 800,000 people in the Clark County metropolitan area and a projected population of 1 million by the year 2000, Las Vegas has become one of the nation's fastest-growing cities. The city generates two-thirds of Nevada's income, mostly from gambling and related tourism.

"We need more water," said Patricia Mulloy, general manager of the Las Vegas Valley Water District, who said the city is growing at the rate of 6,000 people a month while 90 percent of the state's current water supplies are used by 6,000 farmers. Mulloy said tapping underground supplies in these agricultural valleys would bring scores of thousands of new jobs and billions of dollars in revenue to Nevada.

But Mike O'Callaghan, a former Nevada governor who is executive editor of the Las Vegas Sun, has argued in front-page columns that the city has taken an unfair and heavy-handed approach to needs of rural counties. He also has proposed that the state prepare a comprehensive plan before allocating underground water to any claimant.

O'Callaghan appears to be a lonely voice of caution in this optimistic city of illusion, which is defying both the national economic downturn and drought fears that have inspired strict conservation measures and soaring water rates in other Southwest communities.

Golf is the most popular outdoor sport here, and the Las Vegas valley boasts a score of golf courses, each using as much as two million gallons of water a day. Las Vegas water consumption is among the highest in the West -- 380 gallons per person daily compared with a little more than 200 gallons in Phoenix and Los Angeles.

Residential areas are dotted with fountains, swimming pools, artificial lakes and rich, green lawns. A waterfall flows constantly at the entrance of the Mirage, one of the Las Vegas gambling strip's most glittery hotels, although the waterfall uses undrinkable and recycled water.

The waterfall nonetheless suggests to visitors that water, the most precious resource in Nevada, is abundant in this arid valley. The gambling industry has expressed concern that suggestions of scarcity might scare away tourists. Business leaders disdain building moratoriums and say finding alternative sources of water supplies is preferable to limiting growth.

Las Vegas takes most of its water from the Colorado River, where Nevada's share is small compared with those of neighboring California and Arizona. Mulloy said it would be unrealistic for Las Vegas to base long-term growth on the unlikely possibility that Nevada would receive additional shares of already over-allocated Colorado River water.

The Las Vegas alternative has been to lay claim to 864,000 acre-feet of water in underground basins as far as 250 miles away from the city and as much as 1,000 feet below the earth's surface. An acre-foot is the amount of water needed to cover an acre a foot in depth.

The basins, remnants from prehistoric times when most of Nevada was a lake, are scattered throughout 20,000 square miles of central Nevada and fed by subterranean rivers in carbonate rock formations extending through half of Nevada, one-third of Utah and an area of southeastern California that includes Death Valley.

U.S. geologists said it is difficult to measure the long-term impact of withdrawing this underground water. Patricia Port, the regional environmental officer for the Interior Department, compared the interlocking system of underground basins to a huge bathtub in which many of the environmental resources depend on waters near the tub's edge.

"We just don't know what will happen if the level of water in the bathtub is lowered appreciably," she said.

Representatives of the National Park Service and the Fish and Wildlife Service are protesting many of the 146 applications. They said fragile wildlife and plant species in the national parks and other environmental havens are likely to suffer most from lowering the tub. Among these are the Moapa dace, a small fish that survived the Ice Age in warm springs east of Las Vegas.

A better-known endangered species -- the inch-long pupfish living in Devil's Hole in the Death Valley National Monument -- also could be affected. A report by National Park Service scientists contends that use of the underground water sought by Las Vegas might dry up natural springs in the national monument, one of the largest units in the national park system.

Mulloy said last week that both sides have engaged in preliminary "posturing" and acknowledged that compromises may be necessary. But she also reiterated the prevailing view here that using the untapped underground water in the valleys to the north is the key to unabated growth of Las Vegas.